The key advantage of observing background galaxies that are
gravitationally lensed
by foreground mass concentrations in the submm waveband is that the
K correction (Fig. 4) acts to
brighten the distant background lensed galaxy as compared with the lens.
This is already very familiar from surveys of lensed radio AGN
(Rusin, 2001),
and is illustrated clearly in Fig. 1,
in which only the central cD galaxy in the lensing cluster
shows any significant submm emission.

In SCUBA cluster lens surveys, both the image separations, and the
extent of the high-magnification
regions are of order 1 arcmin, a scale which is well matched both to
the 15-arcsec resolution of the JCMT and to the 2.5-arcmin field of view of
SCUBA. The magnification ensures that a significantly greater fraction of
the submm-wave background radiation intensity is thus
resolved into detectable galaxies in surveys in the fields of gravitational
lensing clusters than in even the deepest blank-field surveys
(Blain et al., 1999a).
However, for background sources lensed by galaxies rather than clusters,
the relevant image separations and
the extent of the high-magnification region are only
of order 1 arcsec, and so cannot be resolved using any single-antenna
telescope. High-resolution submm observations are required to
disentangle lensed and unlensed galaxies; this capability will be
provided by ALMA
(Blain, 2002),
while pilot studies of
should be possible using the CARMA, SMA and IRAM PdBI interferometers.
The most luminous lensed sources can already be resolved into multiple
images using the IRAM mm-wave interferometer
(Alloin et al., 1997).

The only caveat for exploiting galaxy-scale lensing is that the
source size must be
small as compared with the area of sky behind the lens that is
strongly magnified. The intense far-IR and submm emission from
low-redshift ULIRGs is typically very compact (several hundred pc across;
Downes and Solomon, 1998),
and would easily meet this condition; however, there are indications
that the dust emission from at least some luminous
high-redshift submm galaxies could extend over
scales greater than 10-kpc
(Papadopoulos et al.,
2001;
Chapman et al., 2001a;
Lutz et al., 2001;
Isaak et al., 2002;
Ivison et al., 2001).
The whole area of sky covered by these galaxies would not then
be lensed efficiently by an intervening galaxy, although bright knots
of emission within them could still be magnified by large factors.
This concern about lensing efficiency and the angular size of distant
submm galaxies does not apply to lensing by much larger
clusters of galaxies, which will always be effective.